I cant say it costs more than normal playing since there are always 2 possibilities.
In normal play, there is no guarantees that you'll get the free spin after losing/wagering equal amount to the feature's price.
While on other side, there is no guarantees that a buy feature will give you a good outcome or at least give what you have paid back.
Personal opinion but imo its much better to buy those bonus round directly instead of trying to get one naturally if you are playing on a high / extreme volatility slot . Its much easier to get one on medium / low volatility but sometimes it might takes over 200 spins to even get one on those high volatility slots
This is personal preference but if you love to watch those spins animation as well as the suspense while playing then its fine though
As Panjul said, it's relatively new feature! And you both understood what I wanted to say/ask!
We can play like crazy and to not get that bonus round... on high volatility slots if you get a bonus in first 100 spins consider yourself lucky, I had situations with a lot more spins than 200, and the best part is when you finally get it, like the punch in the head, you get nothing, like x10!
There are no guarantees that the bonus will be generous, however you come to that bonus, spinning or buying it!
I like to buy bonus rounds, as Arallmuus says, if you like to watch animation, if you like to have higher chances for big winning, and you don't wish to wait for it (hope for it as you spinning and losing money) you will find it's a great feature!
A short example is: You have 7 dollars, and you can play slots with 10c... so you have 70 rounds (a bit more actually, you can have some nice wins along the way) to win the bonus round, or you can buy $7 bonus on Snake and have a chance to win over x1000... lately when I think about it, I rather buy bonus and take my chances!